Bra patterns do exist for purchase, but I made a pattern from an existing bra. Some of the pieces were traceable, but the cup pieces weren't because of the underwires (I did not want to ruin the original by taking the underwires out). So my pattern was approximate, and I did make a test bra and revise the pattern according to how it fit.
There are four pattern pieces:
- upper cup
- lower cup
- side/back
- front stay
The important things about the pattern are a) how the cup pieces fit together, b) the lines of stretch, and c) some edges need seam allowances and some do not. Your fabric will almost certainly be stretchier in one direction than in the 90 degree direction; the stretchiest direction should go up and down on the cup pieces and the front stay, but sideways on the side/back pieces. Check out the pictures for the stretch direction markings.
The cup pieces need seam allowance except for the top of the upper cup (you have a couple options there, see the Cups step). The side/back piece only needs seam allowance at the edge that attaches to the cups, as the rest of the edges will be finished with elastic.
I'm not adding the bra pattern here because it will almost certainly not fit you, you'll have to do a bunch of alteration on it anyway and it's really hardly any more work to trace it from an existing bra that you know fits (or know exactly how it could fit better).
Very nice and useful instructable!